1. Field of the Invention
The present invention relates to an image recording apparatus, etcetera, such as printer, facsimile, copying machine, and specifically to a color image recording apparatus for carrying out a color recording by using line inkjet heads, each of which comprising a plurality of ink ejection nozzles, by the number of heads corresponding to that of colors.
2. Description of the Related Art
Conventionally, inkjet color printers (i.e., color image recording apparatuses) have generally been categorized into two types in terms of difference in recording operations of the inkjet head against a recording sheet at recording thereon.
One type is called a serial printer in which an inkjet head travels back and forth in a horizontal scan direction while a recording sheet is transported in a vertical scan direction.
The other type is called a line printer in which an inkjet head (“line head” hereinafter unless otherwise noted) is furnished fixedly in the main apparatus across the length of an approximate width of the recording sheet and the recording sheet runs in a direction of paper feeding crossing the line head.
In a line printer capable of color recording among the above noted line printers, four color inks (i.e., K for black, C for cyan, M for magenta, and Y for yellow) are usually used, with one line head for each color being furnished. And these four line heads are furnished serially in the direction of transporting the recording sheet.
In such a color line printer, the position of each line head per color must be finely adjusted in order to position each color with one another. Otherwise an image quality will be remarkably reduced, resulting in recognition of a blurred image for its entirety.
An adjustment of each of the above noted line heads where they are mounted must be such that four color recording dots constituting one pixel of an image land on the respective same positions of the recording sheet. This would be easy for a line head with a low resolution made up with a nozzle array pitch of 5 dots per millimeter for instance. However, adjusting line heads of high resolution in a high precision, with a nozzle array pitch of anywhere between one micron and tens of microns (i.e., is to 10 s micrometers) for instance, will be faced by a great technical difficulty.
This will require a great deal of effort and time in assembling a color printer, bringing forth a constraint on improving the assembly process efficiency.
In order to solve such a problem, a technique has been proposed to reduce a positional displacement of recording dots of four color recording dots in a serial inkjet printer to one half pitch or less by making a storing position of image signal variable within the recording buffers of respective colors based on the recording head for the color specified for the most downstream in the slow scan direction (e.g., refer to a Japanese patent laid-open application publication No. 05-330088, paragraph [0047] and FIG. 4).
If a cyan recording dot is displaced leftward by almost one half pitch vis-á-vis a black recording pitch position and a magenta recording dot is displaced rightward by that much vis-á-vis the black, however, the center distance between the cyan and magenta recording dots will end up with about one pitch.